Nitride semiconductors such as GaN, AlN, InN or a mixed crystal thereof have wide band gaps and are used for high output electronic devices and short wavelength light emitting devices. Among the high output electronic devices, technologies for FET (Field Effect Transistor, particularly, HEMT (High Electron Mobility Transistor) are being developed (see, for example, Japanese Laid-Open Patent Publication No. 2002-359256). A HEMT using a nitride semiconductor is used for high output/high efficiency amplifiers, high power switching devices, and the like.
The HEMT used for such devices is required to have characteristics such as “normally-off” and high insulation resistance. From the standpoint of safety, the normally-off characteristic is significant for the HEMT. Various methods for attaining the normally-off characteristic are being considered. One method for attaining the normally-off characteristic is to form a gate recess by removing a portion of a semiconductor layer immediately below a gate electrode (gate recess structure). The gate recess structure has advantages such as making a threshold voltage positive without increasing resistance components in-between electrodes. Furthermore, MIS (Metal Insulator Semiconductor) structures, which include an insulating film used as the gate insulator, are used for horizontal type FETs or HEMTs because high drain resistance and high gate resistance are required for normally-off semiconductor devices used for electric power purposes. Accordingly, by combining the gate recess structure and the MIS structure in an HEMT including a GaN type semiconductor material, the HEMT can be used as a semiconductor device suitable for electric power purposes.
In a case where a GaN type semiconductor material is used in the HEMT having a MIS structure, the GaN type semiconductor material exhibits a strong piezo polarization and spontaneous polarization and has an extremely high electron density in the semiconductor layer (electron transit layer). Therefore, the threshold voltage may not shift much toward the positive even by forming a gate recess in the HEMT. Thus, the HEMT may not be able to attain a normally-off characteristic by merely forming the gate recess.
Further, in a case of manufacturing HEMTs where the gate recesses are formed by dry-etching with a gas containing a chlorine component, the depths of the gate recesses become inconsistent. This leads to inconsistent characteristics and low yield of the manufactured HEMTs. Further, the bottom surface of the gate recess tends to become bumpy (concavo-convexo) by using the dry-etching method. This causes electrons to be unexpectedly captured in the trap level. Thus, when electrons are captured in the trap level, the threshold voltage changes. As a result, it becomes difficult for the HEMTs to attain steady operating characteristics.